1. Field of the Invention
The present invention relates to an optical receiver having a function for suppressing useless intensity modulation components. More particularly, the present invention relates to an optical receiver provided on a transmitting terminal in a system for amplifying an optical signal to perform multi-repeating.
2. Description of the Related Art
In a system, which amplifies and repeats optical signals by multi staged amplifying repeaters, it is well-known that SN ratio of the optical signals received in a receiver terminal can be deteriorated where there is a polarization dependency in a gain of an optical amplifier. The gain polarization dependency is provided at each of the amplifying repeaters.
An Erbium Doped Fiber (EDFA), which belongs to rare-earth doped fiber amplifiers, has a polarization Hole Burning (PHB) effect. The PHB effect of the EDFA, is a main factor in causing gain polarization dependency.
To suppress the SN ratio deterioration, if polarization scrambling for changing input polarization condition in a speed higher than response speed of the PHB, i.e., modulating polarization, is performed, the effect of PHB can be reduced.
However, it is known that polarization modulation is converted to intensity modulation to generate useless intensity modulation components because of polarization dependency loss of passive parts employed in an optical amplifier, when the polarization scrambling is performed. For example, such the problem is described in "Polarization Hole-Burning in Erbium-Doped Fiber-Amplifier Transmission Systems" in ECOC '94, Proceedings Vol. 2 P621-628.
On the other hand, in an optical amplifying multi-repeaters system, a monitoring control signal is transmitted to each of the repeaters from a transmitter terminal to perform a monitoring control for the system. To realize the transmission system, a monitor control signal is piled to a main signal by employing several percentages of an intensity modulation.
FIGS. 1 and 2 are diagrams for explaining a structure of a conventional optical receiver and the above-described problems.
In FIG. 1, an optical receiver is formed by an optical pre-amplifier 1, an optical to electrical converter 2 for a main signal, electrical amplifiers 3 and 4, a timing extracting circuit 5, and an identifying device 6. The optical pre-amplifier 1 includes a laser optical source 10 for excision and a rare-earth-doped fiber 11, may be an Erbium doped fiber, for example.
The rare-earth doped fiber 11 is formed as a traveling-wave type laser, in which a rare earth element ion makes an optical excision according to the difference of an energy level. A semi-conductor laser having high output power is preferably employed as laser optical source 10.
An optical received signal amplified in optical pre-amplifier 1 is inputted to optical to electrical converter 2 for a main signal and converted to an electrical signal. The electrical signal converted in optical to electrical converter 2 for a main signal, is amplified at predetermined times, in electrical amplifiers 3 and 4, and inputted to an identifying device 6.
The timing extracting circuit 5 extracts a signal timing of the electrical signal branched from the amplifier 3. The identifying device 6 identifies the electrical signal by synchronizing the extracted signal timing.
FIG. 2 shows a frequency band characteristic of the optical receiver shown in FIG. 1. In FIG. 2, the axis of abscissas shows an intensity modulating frequency of an inputted optical signal, and the axis of ordinates shows a signal gain. In FIG. 2, (I) shows a frequency gain characteristic of a main signal.
A cut off frequency in the frequency gain characteristic of the main signal is set to extent 0.7 times of a bit rate of the main signal. Meanwhile, (II) shows a spectrum of the above-described modulating frequency fscr for polarization scrambling and monitor control signal modulating frequency fsv. The modulating frequency fscr for polarization scrambling and monitor control signal modulating frequency fsv are extent 10 KHz, for example, for a 10 GHz bit rate of the main signal. Accordingly, in FIG. 2, a spectrum for the fscr and fsv is shown as one common spectrum.
In this respect, in the conventional optical receiver, an intensity modulating component due to the modulating signal for polarization scrambling fscr or monitor control signal fsv is within the frequency characteristic band of the optical receiver. As a result, noise associated with the main signal is increased and becomes a factor in the deterioration of a transmission characteristic.